Method for monitoring the functioning of a NOx sensor arranged in an exhaust gas channel of an internal combustion engine

ABSTRACT

The invention relates to a method for monitoring the functioning of an NOx sensor, which is arranged in an exhaust gas channel of an internal combustion engine and is located downstream from an NOx storage catalytic converter. The aim of the invention is to detect the faulty functioning of the NOx sensor in a simple manner in order to be able to take appropriate measures if necessary. To this end, the mass of NOx absorbed by the NOx storage catalytic converter is determined within a diagnostic period using a measurement signal of the NOx sensor. At the same time, an absorbed NOx target mass is calculated using a model for the NOx storage catalytic converter. A ratio of the NOx mass to the NOx target mass is then compared to predetermined limit values to determine the functioning of the NOx sensor. The aim of the invention is alternatively accomplished by measuring a duration time for a complete NOx regeneration of the NOx storage catalytic converter. A specified duration time is also calculated for the NOx regeneration using a model for the NOx storage catalytic converter and using a measured or calculated NOx load state. A ratio of the measured duration time to the specified duration time is then compared to predetermined limit values to determine the functioning of the NOx sensor.

BACKGROUND OF THE INVENTION

The invention relates to a process for monitoring the function of an NOxsensor arranged in an exhaust duct of an internal combustion engine. Toreduce the emission of pollutants from an internal combustion engine, aknown practice is to arrange suitable catalysts in the exhaust gas ductof the engine. The catalyst collects pollutants, such as CO, HC or H₂that can act as reducing agents and be oxidized by atmospheric oxygen.These reducing agents may also react with the NOx, produced during thecombustion process in the engine, to form nitrogen.

If the engine is operating in a lean mode the proportion of oxygen inthe air-fuel mixture is increased, which is more favorable tocombustion, and as a consequence the proportion of the reducing agentsin the exhaust will decrease. In this case, an adequate reaction of NOxon the catalyst will no longer be ensured. As a remedy, an NOx reservoirmay be arranged in the exhaust duct and combined with the catalysts tomake an NOx storage catalyst. The NOx storage catalyst will absorb NOxif the NOx desorption temperature is not exceeded or until the NOxstorage capacity is reached. Prior to reaching it's storage capacity,the system will have to change to a regeneration mode to regenerate theNOx storage catalyst by a period of rich operation, and prevent NOxemission.

To determine whether regeneration is necessary the NOx concentrationdownstream from the NOx storage catalyst may be detected with an NOxsensor. A disadvantage of this, however, is that if the NOx sensormisfunctions, high NOx emissions may occur, or NOx may be unnecessarilyconsumed during a premature regeneration. The object of the presentinvention is to detect misfunctions of the NOx sensor in a simple mannerso appropriate countermeasures may be adopted if necessary.

SUMMARY OF THE INVENTION

The object of the invention is accomplished by a method of monitoringthe functioning of an NOx sensor. During a diagnostic period the levelsof NOx not absorbed by the NOx storage catalyst are detected by thesensor and accumulated. From the levels detected by the sensor the massof NOx absorbed by the storage catalyst during this diagnostic periodmay be determined. At the same time, a target mass of NOx absorbed maybe calculated using a model of the NOx storage catalyst. The ratio ofthe detected NOx mass absorbed, to the NOx target mass calculated isthen determined and compared to a lower bound and an upper bound. If themass ratio falls below the lower bound or above the upper bound amaintenance signal may then be generated.

In a second embodiment, the function of an NOx sensor is monitored bydetermining a duration time for a complete NOx regeneration of the NOxstorage catalyst. A target duration time for a complete NOx regenerationmay then be determined from a model of the NOx storage catalyst and ameasured or calculated NOx loading condition. The ratio of the durationtime determined for a complete NOx regeneration, to the measured orcalculated target duration time is then calculated and compared to alower bound and an upper bound. If the time ratio falls below the lowerbound or above the upper bound a maintenance signal may then begenerated.

After the occurrence of a maintenance signal, in either situation, theerror can be corrected by suitable measures, or the NOx sensor can bereplaced if necessary. Further, it is advantageous to set the period ofdiagnosis to begin immediately after a complete NOx regeneration of theNOx storage catalyst and a change to a lean mode of the engine.Preferably, the period of diagnosis will end after identification of aneed to regenerate the NOx storage catalyst or upon a change inregeneration mode. Monitoring the function of the NOx sensor shouldpreferably take place only when a selected period of constant lean modeof operation of the engine has been detected. In this way, theinfluences from the dynamic operation of the engine on the model of thestorage catalyst, that are difficult to allow for, can be avoided.

For a better understanding of the present invention, together with otherand further objects thereof, reference is made to the followingdescription, taken in conjunction with the accompanying drawings, andits scope will be pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram depicting the arrangement of an internalcombustion engine having an exhaust duct, a precatalyst, a NOx storagecatalyst and an NOx sensor.

FIG. 2 is a flow diagram of a method for monitoring the function of theNOx sensor according to an embodiment of the invention.

DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 are drawings showing embodiments of the invention. FIG. 1shows an arrangement of an internal combustion engine 10, including anexhaust duct 12, a precatalyst 14, and a NOx storage catalyst 16. Theprecatalyst 14, and the NOx storage catalyst 16, serve to diminishpollutant emission of the engine 10. Ordinarily, the catalysts 14 and 16include components that make possible the oxidation of reducing agentsformed during combustion, such as CO, HC or H₂, by atmospheric oxygen.At least the NOx storage catalyst 16, comprises a catalyst componentthat makes possible the reduction of NOx, formed during the process ofcombustion of an air-fuel mixture, by means of the reducing agents. Ifthe engine 10, is in a lean mode of operation, as a rule, the proportionof reducing agents in the exhaust will not be sufficient to ensure ahigh conversion of NOx. In lean mode, therefore, the NOx will beabsorbed as nitrate by the storage component of the NOx storage catalyst16.

The absorption of the NOx can take place only until either an NOxdesorption temperature is exceeded or the NOx storage capacity isexhausted. Before this point, therefore, the system must change into aregeneration mode, with an air to fuel ratio ≦1, to cause NOxregeneration. Whether there is a need for regeneration will bedetermined by the NOx concentration or emission detected by the NOxsensor 18. A test signal will be passed on to an engine controlinstrument 20, evaluated, and used to control the operating mode of theengine 10.

FIG. 2 shows a block diagram which depicts a monitoring of the functionof the NOx sensor 18, during dynamic operation of the engine 10. Forexample, In step S1, it is first detected whether a complete NOxregeneration of the NOx storage catalyst 16, has been carried out. Ifthis is not the case, then the monitoring of the function of the sensor18, is discontinued (step S2). If complete regeneration of the NOxstorage catalyst has been carried out a lean mode will begin step S3.

At the beginning of the lean mode (step S3), a determination of an NOxmass absorbed in the NOx storage catalyst 16, is started. During apreassigned period of diagnosis, the NOx concentration downstream fromthe NOx storage catalyst 16, is detected by the NOx sensor 18. Thisdetected concentration is accumulated, and then deducted from a measuredor calculated crude NOx emission of the engine 10. Second, with the aidof known models of the NOx storage catalyst 16, and from the crude NOxemission, a target mass of absorbed NOx is calculated. The target NOxmass corresponds to a maximum NOx mass that can be absorbed by a freshNOx storage catalyst 16.

In step S4, the engine 10, is checked continuously to determine whetherit is in constant lean operation during the period of diagnosis. In thecase of disturbances due to dynamic processes, for example a change in ahomogeneous mode or an abrupt shutdown, the target NOx mass calculatedfor the period of diagnosis is especially prone to error, and thereforethe monitoring of function is broken off (step S5). Preferably, theperiod of diagnosis is determined so that it begins with the change inthe lean mode (step S3) and continues until a need for regeneration isdetected (step S6).

Such a need for regeneration may for example be detected by way of theNOx sensor 18, in the form of a threshold emission for NOx. Once theneed for regeneration is present, a change into the regeneration modewith an air to fuel ratio ≦1 is initiated (step S7). Simultaneously, atime counter is started, to determine a duration time for a complete NOxregeneration.

From the absorbed NOx mass found by way of the NOx sensor 18, for theNOx storage catalyst 16, and the target NOx mass, a mass ratio isdetermined in a step S8. In step S9, if the mass ratio is above an upperbound or below a lower bound, then there is a defect in the NOx sensor18, and a maintenance signal is generated (step S10). The upper massratio bound usually reflects a ratio of the NOx mass found by way of theNOx sensor 18, to the target NOx mass in a fresh NOx storage catalyst16.

If the mass ratio lies between the two bounds, then in a step S11 it canbe checked whether the NOx regeneration has been carried out completely.For this purpose, for example, a probe 22, that measures the air to fuelratio, arranged downstream from the NOx storage catalyst 16, issuitable. Towards the end of the NOx regeneration, the value of the airto fuel ratio declines distinctly, and for example by preassignment of asuitable threshold value, a stop signal can be set for the time counter(step S13). If the NOx regeneration is discontinued prematurely, thefunctional monitoring of the NOx sensor 18, will be broken off in stepS12.

With the aid of the model of the storage catalyst, a target durationtime for the NOx regeneration is calculated from a measured orcalculated state of NOx loading. In step S14 a time ratio will bedetermined by taking the ratio of the duration time, determined in stepS7, to the target duration time. In step S15 the time ratio is comparedwith an upper bound or a lower bound. If the time ratio is above theupper bound or below the lower bound, there is a sensor defect, and amaintenance signal is generated (step S16). If this is not the case,then a new cycle of functional monitoring, beginning with step S3, maybe initiated. The upper bound is again so chosen that it reflects aratio of the duration time to the target duration time in a fresh NOxstorage catalyst 16. Sensor plausibility is also checked to determinewhether a lesser measure of storage fill is yielded, for example, as mayoccur with poorer storage behavior of the catalyst or when the measuredregeneration time required is reduced to a corresponding extent.

While there have been described what are believed to be the preferredembodiments of the invention those skilled in the art will recognizethat other changes and modifications may be made thereto withoutdeparting from the spirit of the invention, and it is intended to claimall such changes and modifications as fall within the true scope of theinvention.

1. A process for monitoring the function of a NOx sensor arranged in anexhaust duct of an internal combustion engine downstream from a NOxstorage catalyst, comprising: (a) monitoring and accumulating NOx levelsdetected by said sensor during a diagnostic period; (b) determining amass of absorbed NOx by said NOx storage catalyst as a differencebetween engine NOx emissions and said accumulated NOx levels detected bysaid sensor; (c) calculating a target mass of absorbed NOx from a modelof said NOx storage catalyst; and (d) determining a mass ratio of saidabsorbed NOx mass to said target NOx mass and comparing said mass ratiowith at least one of a lower bound and an upper bound.
 2. A processaccording to claim 1, further comprising comparing said mass ratio toboth said lower bound and said upper bound and generating a maintenancesignal when said mass ratio is below said lower bound and generating amaintenance signal when said mass ratio is above said upper bound.
 3. Aprocess according to claim 1, wherein said diagnostic period beginsimmediately after a complete NOx regeneration of said NOx storagecatalyst and a return to a lean mode of operation of said internalcombustion engine.
 4. A process according to claim 3, wherein saiddiagnostic period ends after identification of a need for regenerationof said NOx storage catalyst.
 5. A process for monitoring the functionof a NOx sensor arranged in an exhaust duct of an internal combustionengine downstream from a NOx storage catalyst which comprises: (a)determining a duration time for a NOx regeneration of said NOx storagecatalyst; (b) calculating a target duration time for said NOxregeneration using a model of said NOx storage catalyst and NOx loading;and (c) determining a time ratio of said duration time to said targetduration time and comparing said time ratio with one of a lower boundand an upper bound.
 6. A process according to claim 5, furthercomprising comparing said time ratio to both said lower bound and saidupper bound and generating a maintenance signal when said time ratio isbelow said lower bound and generating a maintenance signal when saidtime ratio is above said upper bound.
 7. A process according to any ofthe preceding claims, wherein said monitoring the function of said NOxsensor takes place only during a selected period of lean operation ofsaid internal combustion engine.